The production of electricity is generally more expensive during peak demand hours than at low demand hours. Therefore, various energy storage systems have been developed which permit the storage of energy for later use. In particular, thermal energy storage systems permit thermal energy to be generated during off-peak periods and stored for use during subsequent on-peak periods. Such deferred use of stored energy can reduce strain on the power grid and/or reduce the average cost of energy per kilowatt-hour during peak load periods. However, some previous thermal energy storage systems suffer from one or more disadvantages, such as short thermal energy storage periods, low capacity, low efficiency, low versatility, and difficulty of installation. Improved thermal energy storage systems are therefore desired.